1. Field of the Invention
The present invention relates to electric circuits, and more particularly, to a shunt-feedback technique for improving noise figure and linearity in electronic circuits such as amplifiers and mixers.
2. Background Art
Noise figure and linearity are among key characteristics of amplifiers and mixers. Noise figure represents the degradation of signal-to-noise ratio, or, SNR (increase in noise), caused by a circuit. (SNR is the ratio of the power of a desired signal to the power of noise signals.) Hence, the noise figure corresponds to the difference between the SNR at the input of the circuit and the SNR at the output of the circuit. Noise figure is especially important for the first stages of an amplifier chain because noise produced in, or passed through, the first stage will be amplified by subsequent stages.
The linearity of a circuit may be defined by various parameters. For example, the input third-order intercept point represents third-order non-linearity in the transfer function of a circuit. Due to the third-order non-linearity, two undesired signals in adjacent channels generate output third-order intermodulation products (IM3) which can corrupt the desired signal at the output. The power of the desired signal at the output of a linear circuit increases linearly with the input power of the circuit. However, the power of the output IM3 increases with the cube of the input power. The input third-order intercept point is the input power level, at which the power of the desired signal at the output of a circuit is equal to the power of the output IM3.
FIG. 1 shows a typical low-noise amplifier (LNA) 10 for amplifying input signals, such as radio-frequency signals propagating through receiving circuitry of a cellular telephone. A signal input RFin is provided for receiving an input signal from a source, such as a tuner or the previous amplifier stage. The input signal is supplied to the base of an NPN junction transistor Q1. A degeneration impedance Ze is coupled between the emitter of the transistor Q1 and a ground terminal. The transistor Q1 and the impedance Ze function as a common-emitter transconductance stage.
An NPN junction transistor Q2 is connected to the transistor Q1 in a cascode configuration. The emitter of the transistor Q2 is connected to the collector of the transistor Q1. The base of the transistor Q2 is supplied with bias voltage from a bias input. A resistor R1 and an inductor L1 are coupled between the collector of the transistor Q2 and a source of collector voltage Vcc. A capacitor C1 is arranged between the collector of the transistor Q2 and an output RFout of the low noise amplifier 10.
The resistor R1 is an output-matching resistor that functions to match the output impedance of the low noise amplifier 10 with the impedance of a load coupled to the output RFout. The inductor L1 and capacitor C1 form an impedance transformation network that transforms the output impedance defined by the resistor R1 to match the impedance of the load. The inductor L1 also serves as a pull-up inductor that increases the allowable voltage at the collector of the transistor Q2.
In some applications, high linearity is as important as low noise figure. In conventional amplifiers, linearity is improved by increasing the impedence value of the degeneration impedance Ze. However, a high degeneration impedance Ze results in a high impedance at the amplifier input RFin. To match the high input impedance of the amplifier 10 with the impedance of the source, an impedance matching network is required between the input RFin and the source.
The sensitivity of the impedance matching network to component variations depends on the impedance mismatch between the amplifier input and the source. The larger the impedance mismatch is, the more sensitive the matching circuit becomes.
Further, an increase in impedance at the base of the transistor Q1 causes an increase in base shot noise contribution from the transistor Q1. (Shot noise is associated with direct-current flow in a pn junction. This current is composed of holes and electrons, which have sufficient energy to overcome the potential barrier of the junction. As the passage of each carrier through the junction is a random event, it causes random fluctuations of the current called the shot noise. Base shot noise is the shot noise of a bipolar transistor caused by the base current.) As a result, the noise figure of the amplifier 10 increases. An increase in the degeneration impedance thus cannot improve the linearity of the amplifier 10 without degrading other performance parameters of the amplifier, such as noise figure.
It would be desirable to create a new technique that improves not only the linearity of an electronic circuit but also its noise figure.